1. Field of the Invention
The present invention relates to a method and an apparatus for molding a ceramic sheet used, for example, for a laminated exhaust gas sensor and a laminated heater.
2. Description of the Related Art
The doctor blade method (casting method) and the extrusion molding method are known for molding a ceramic sheet.
Generally, a ceramic sheet is molded by the doctor blade method. In this method, ceramic powder is mixed with an organic binder, a solvent, etc. to produce a slurry. The slurry is injected into a dam having a doctor blade arranged on a carrier film, and the carrier film is moved at a predetermined speed in a predetermined direction. As a result, the slurry flows out continuously from the gap between the doctor blade and the carrier film. After that, the slurry is dried together with the carrier film and separated from the carrier film thereby to produce a ceramic sheet of a predetermined thickness.
The doctor blade method, however, requires the use of a slurry with a large amount of solvent added thereto, and therefore a large number of pores are formed in the dried sheet after volatilization of the solvent. The presence of a large number of pores causes such inconveniences as a reduction in the ceramic powder filling factor, an increased burning shrinkage and variations of the burning shrinkage. This poses the problem of variations of the product size.
It is very difficult to maintain the thickness of a slurry. Therefore, a thick sheet cannot be molded from a slurry. When manufacturing a product using a thick sheet, therefore, the problem is posed that an appropriate number of thin sheets are required to be stacked.
Also, it is very difficult to acquire monodisperse ceramic powder having a uniform grain size distribution. Therefore, the grain size distribution of the ceramic powder has a certain margin. Even if a thick sheet could be molded by the doctor blade method, ceramics of larger grains naturally sediment faster in the slurry in the drying process. Thus, a density difference occurs between the upper surface portion and the lower surface portion of the sheet. As a result, a difference of the burning shrinkage occurs between the upper and lower surfaces, thereby posing the problem of a warped product. This problem may be caused also with a thin sheet processed by the doctor blade method.
In the extrusion molding method, on the-other hand, the filling factor of ceramic powder is so high that a thick sheet can be molded.
The extrusion molding is of two types, plunger and screw (auger). The extrusion molding of a plunger type is a method in which a ceramic material is filled in a mold and extruded from the mold by piston, and can produce a predetermined fluidity depending on the manner in which the ceramic material is filled. Nevertheless, the disadvantage of this method is that the ceramic material cannot be extruded continuously.
The extrusion molding of screw type, on the other hand, is a method in which the ceramic material is continuously extruded from a mold by rotating a screw. Due to the variations of fluidity of the ceramic material in the screw extruder, however, an attempt to mold a wide, thin sheet using an extruder having a screw of a small diameter would partially increase or decrease the molding pressure, resulting in an irregular flow of the ceramic material which in turn leads to wrinkles in the sheet.
Various solutions to this problem have been proposed as described below.
(1) The pressure exerted on the ceramic material is sufficiently equalized by increasing the screw diameter. This solution can prevent the generation of wrinkles on the sheet but leads to a very bulky screw extruder. As a result, when changing the ceramic material, the disassembly and cleaning process requires a great number of steps. Also, since a greater amount of ceramic materials are left in the apparatus, the yield of the material is considerably deteriorated.
(2) Japanese Unexamined Patent Publication No. 63-307903 discloses a technique for the plunger type, in which the flow rate of the sheet is substantially equalized at the ends and the central portion of the sheet by setting the temperature higher at the ends than at the central portion of the sheet. For the screw type of extrusion molding, however, unlike the plunger version, the flow rate at the central portion is not always high, and therefore the flow rate adjustment is impossible in the case where the flow rate at the ends or a given portion is higher. Therefore, this technique is not directly applicable to the screw type of extrusion molding. Even if applicable, many defects would be caused in the sheet in the case where the screw diameter is large.
(3) Japanese Unexamined Patent Publication No. 61-125805 proposes a technique for regulating the flow rate by extending or retracting a rectification block. However, this is intended for applications to thick, wide sheets and fails to achieve the object of the present invention.
(4) Japanese Unexamined Patent Publications No. 9-328366 and No. 10-152379 propose a technique in which the fluidity of the ceramic material (body) can be improved by changing the plasticizer or the like added to the body to produce a uniform sheet. This method, however, poses the problem that a change in the composition of the additive changes the various ceramic characteristics including the burning shrinkage resulting in different product performance.
In spite of the various techniques thus far proposed as solutions to prevent the wrinkling of a ceramic sheet as described above, an effective solution for the screw extruder has yet to be discovered. Especially, a method has not yet been established for the extrusion molding of a wide, thin ceramic sheet in the screw extruder which can suppress wrinkling.